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Re: [OM] Astrophotography

Subject: Re: [OM] Astrophotography
From: "O.C" <oben@xxxxxxxxx>
Date: Mon, 13 Aug 2001 16:25:51 +1000

What Roger is talking about is the absolute diameter, not the f-number. Thus, if I have the following lenses, the absolute aperture diameters, wide open are:
...
  100/2   = 50.0mm
  135/2.8 = 48.2mm
  200/4   = 50.0mm
  300/4.5 = 66.7mm

Hmm makes my 180 f2... very interesting...

Absolute aperture:
-> 90mm
... wow.

Amount of light multiplication factor / ratio:
-> 6.36 x
another wow...

Equivalent exposure:
-> 6.36x ~7 = 44.5 seconds

Better than the 50/1.2 even. Nice exercise in calculation actually. I've seen a photograph of Orion's belt with a 180/2.8 and even that was impressive in that a small black area corresponding to the Horsehead nebula was distinctly visible as was the Orion nebula. I'll have to try the 180/2 out.

Even more promising seems the fact that the 180/2 has absolute chromatic aberration correction (it has no infrared mark even), so that star images should be even more pointlike and not affected by a red ring effect.

Thanks for the inspiration John.


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